Experiment 320: Probing Strong-Field QED by Colliding Short Laser Pulses with Ultra-Relativistic Electron Beams

Impressive progress in extreme-light technologies has made multi-PW lasers promising tools for generating ultra-strong electromagnetic fields. Nonetheless, they are still unable to probe the Schwinger field directly. Reaching this scale in a terrestrial laboratory requires exploiting the Lorentz boost provided by ultra-relativistic electron beams. In nearly head-on collisions, a relativistic electron experiences the laser intensity enhanced by $\sim 4\gamma^2$ in its rest frame, where $\gamma$ is the Lorentz factor. At the Schwinger critical field, $E_{\text{cr}} = m^2c^3/(\hbar e)$, an electric field is expected to generate matter–antimatter pairs by separating virtual pairs arising from quantum fluctuations.

In this talk, we will present recent progress from Experiment 320 at SLAC’s FACET-II linear accelerator. In particular, we will discuss how we determine and actively stabilize electron–laser collisions near the optimal relative arrival time of the two beams. In the presence of temporal drifts on the picosecond timescale, measuring the relative arrival time on a shot-to-shot basis is essential. E-320 accomplishes this using electro-optical sampling (EOS), combined with real-time Bayesian analysis of the scattered-electron spectra, thereby maximizing the number of shots that interact with the laser pulse at focus.

We will present results on the transition from the perturbative multiphoton regime to the strong-field quantum regime of nonlinear Compton scattering. In the perturbative regime, electrons interact with individual laser photons, whereas in the strong-field regime they experience the collective electromagnetic field of the laser through interactions with many coherent photons. Consequently, a semiclassical description becomes applicable in the strong-field regime: electrons experience the classical force of the laser and oscillate in its periodic field much like in a magnetic undulator. Their emission spectrum should therefore be described by classical synchrotron radiation theory. The critical photon energy, which characterizes the peak of the synchrotron spectrum, is given by $\hbar\omega_c \sim \chi \epsilon$, where $\epsilon$ is the energy of the emitting electron, $\chi = E^*/E_{\text{cr}}$ is the quantum parameter, and $E^*$ is the electric field strength in the electron rest frame. As the recoil induced by single-photon emission becomes substantial for $\chi \gtrsim 0.1$, significant quantum corrections to the classical radiation theory are expected in this regime.

Importantly, the origin of the recoil in the strong-field quantum regime differs from the recoil experienced in the perturbative regime, where the electron undergoes essentially ballistic scattering with individual photons and the recoil is a direct consequence of energy-momentum conservation. In the strong-field regime, however, the electron both absorbs and re-emits photons into the laser modes, resulting in a much more complicated dynamic. We have experimentally demonstrated the transition to the strong-field quantum regime by first observing a redshift of the individual harmonics, characteristic of the perturbative regime, followed by their gradual merging into a quasi-continuous, exponentially decaying synchrotron-like spectrum. Since we still observe substantial recoil in the strong-field regime ($\chi \gtrsim 0.3$), while the probability for multiple-photon emission remains small, our observations can only be explained by incorporating quantum modifications to the definition of the critical synchrotron frequency.

Finally, we will provide an update on preparations for measuring electron–positron pair production in the deep-tunneling regime.